The invention relates to a write-once optical data carrier comprising a cyanine dye as light-absorbent compound in the information layer, to a process for its production and also to the application of the above-mentioned dyes to a polymer substrate, in particular polycarbonate, by spin coating or vapour deposition.
Write-once optical data carriers using specific light-absorbent substances or mixtures thereof are particularly suitable for use in high-density writeable optical data stores which operate with blue laser diodes, in particular GaN or SHG laser diodes (360-460 nm) and/or for use in DVD-R or CD-R disks which operate with red (635-660 nm) or infrared (780-830 nm) laser diodes.
The write-once compact disk (CD-R, 780 nm) has recently experienced enormous volume growth and represents the technically established system.
The next generation of optical data storesxe2x80x94DVDsxe2x80x94is currently being introduced onto the market. Through the use of shorter-wave laser radiation (635-660 nm) and higher numerical aperture NA, the storage density can be increased. The writeable format in this case is DVD-R.
Today, optical data storage formats which use blue laser diodes (based on GaN, JP 08 191 171 or Second Harmonic Generation SHG JP 09 050 629) (360nm -460 nm) with high laser power are being developed. Writeable optical data stores will therefore also be used in this generation. The achievable storage density depends on the focussing of the laser spot on the information plane. Spot size scales with the laser wavelength xcex/NA is the numerical aperture of the objective lens used. In order to obtain the highest possible storage density, the use of the smallest possible wavelength xcex is the aim. At present 390 nm is possible on the basis of semiconductor laser diodes.
The patent literature describes dye-based writeable optical data stores which are equally suitable for CD-R and DVD-R systems (JP-A 11 043 481 and JP-A 10 181 206). To achieve a high reflectivity and a high modulation height of the read-out signal and also to achieve sufficient sensitivity in writing, use is made of the fact that the IR wavelength of 780 nm of CD-Rs is located at the foot of the long wavelength flank of the absorption peak of the dye and the red wavelength of 635 nm or 650 nm of DVD-Rs is located at the foot of the short wavelength flank of the absorption peak of the dye. In JP-A 02 557 335, JP-A 10 058 828, JP-A 06 336 086, JP-A 02 865 955, WO-A 09 917 284 and U.S. Pat. No. 5,266,699, this concept is extended to the 450 nm working wavelength region on the short wavelength flank and the red and IR region on the long wavelength flank of the absorption peak.
Apart from the abovementioned optical properties, the writeable information layer comprising light-absorbent organic substances has to have a substantially amorphous morphology to keep the noise signal during writing or reading as small as possible. For this reason, it is particularly preferred that crystallization of the light-absorbent substances be prevented in the application of the substances by spin coating from a solution, by vapour deposition and/or sublimation during subsequent covering with metallic or dielectric layers under reduced pressure.
The amorphous layer comprising light-absorbent substances preferably has a high heat distortion resistance, since otherwise further layers of organic or inorganic material which are applied to the light-absorbent information layer by sputtering or vapour deposition would form blurred boundaries due to diffusion and thus adversely affect the reflectivity. Furthermore, a light-absorbent substance which has insufficient heat distortion resistance can, at the boundary to a polymeric support, diffuse into the latter and once again adversely affect the reflectivity.
A light-absorbent substance whose vapour pressure is too high can sublime during the above-mentioned deposition of further layers by sputtering or vapour deposition in a high vacuum and thus reduce the layer thickness to below the desired value. This in turn has an adverse effect on the reflectivity.
It is therefore an object of the invention to provide suitable compounds which satisfy the high requirements (e.g. light stability, favourable signal/noise ratio, damage-free application to the substrate material, and the like) for use in the information layer in a write-once optical data carrier, in particular for high-density writeable optical data store formats in a laser wavelength range from 340 to 830 nm.
Surprisingly, it has been found that light-absorbent compounds selected from the group of cyanine dyes can satisfy the abovementioned requirement profile particularly well.